The known risks associated with exposure to radiation is a key motivator for the development of new detector technologies able to produce better images using lower patient exposures. In diagnostic radiology, lower patient exposures generally result in fewer x-ray quanta interacting in the detector and reduced image signal-to-noise ratio (SNR). Image SNR, expressed as a spatial-frequency-dependent noise-equivalent number of quanta (NEQ) determines what can and cannot be seen in a noise-limited image. For example, a recent comparison of diagnostic accuracy using computed radiography (CR) and flat-panel digital radiography (DR) in a breast cancer screening program found DR had a better cancer detection rate compared to CR, attributed to a difference in system modulation transfer function (MTF) and image SNR. The detective quantum efficiency (DQE), initially called the equivalent quantum efficiency, describes the NEQ for a given number of x-ray quanta incident on a detector.
The DQE of an ideal detector would be equal to the quantum efficiency (fraction of x-ray quanta that interact in the detector) for all spatial frequencies of importance. However, in practice the DQE always decreases further with increasing frequency due to a number of considerations, including: resolution loss as described by the MTF; scatter of secondary quanta (optical scatter in a phosphor or charge migration in a photoconductor); reabsorption or escape of characteristic emissions from photo-electric interactions and Compton scatter; and noise aliasing. Noise aliasing is the primary cause of DQE degradation with frequency for a-Se detectors at mammographic energies, reducing the SNR for visualizing small lesions and fine image details.
Therefore, there is provided a novel method and apparatus for improved modulation transfer function and detective quantum efficiency in an X-ray detector.